This invention relates generally to the field of wireless communications systems and, more particularly, to apparatus and methods for recovering carrier phase of multi-rate signals.
In typical wireless communication systems a Cell Site Modem (CSM) is used for communication between the base station and the mobile station. Among other things, the CSM recovers the carrier phase of the link signal from the mobile to base station. Carrier phase recovery (sometimes referred to as phase referencing) is the operation of extracting a phase coherent reference carrier from a received carrier.
The base station transmits a pilot channel as a reference channel. This allows the mobile station to acquire the timing of the forward channel and thus provides a phase reference for the mobile station. However, the IS-95 standard does not provide for the mobile station transmitting a reference channel to the base station. Therefore the base station must use the link signal to estimate the carrier phase.
Generally, CSMs use non-coherent demodulation. The drawback of this method is the signal to noise ratio degradation at each demodulator. In addition, non-coherent demodulation prevents rake receivers from using more sophisticated combining methods to achieve higher combining gain for a multipath fading channel.
Coherent demodulation provides a better signal to noise ratio than non-coherent techniques. However, it is difficult to recover carrier phase when the link signal is the only signal to work from. Carrier recovery used for coherent demodulation is complicated further by the fact that IS-95 uses a data burst randomizer to transmit multi-rate data.
When the signal received by the base station is at the full rate, conventional systems and methods of carrier recovery for coherent demodulation may be employed. However, when the signal received by the base station is a series of random bursts, each burst having the length of one power control group (i.e. 6 Walsh symbols in IS-95), conventional CSMs do not effectively estimate carrier phase for use in coherent demodulation. This is because conventional CSMs do not perform complex (i.e. magnitude and phase) channel gain estimation on a single power control group. Thus, sufficient gains are difficult to achieve for the different data rates.
Others have attempted to recover carrier phase from the mobile to base station link signal using phase locked loop systems. However these systems do not provide adequate carrier phase. Phase locked loop systems require continuous signal input and cannot operate at lower transmission rates when signals become intermittent. While more gain may be achieved for full rate transmissions, losses can occur for other lower transmission rates. Systems which utilize tentative non-coherent demodulation and moving average complex estimation also require continuous signal input and experience similar problems at lower rate transmissions.
Other proposals which utilize aided symbol or aided signal technologies require modification of IS-95 standard devices and methods. These proposals would be incompatible with the existing wireless communication infrastructure.
Accordingly there exists a need for systems and methods of performing coherent complex channel gain estimation on link signals which is effective for signals transmitted by IS-95 burst randomization.
There also exists a need for such systems which operate on a power control group.
Accordingly it is an object of the present invention to provide systems and methods for performing coherent complex channel gain estimation on link signals which is effective for signals transmitted by IS-95 burst randomization.
It is also an object of the present invention to provide systems and methods which operate on a power control group.
In accordance with the teachings of the present invention, these and other objects may be accomplished by the present invention, which is a system for performing complex channel gain estimation from a transformer output and a non-coherent combiner output. A selector determines an orthogonal function index from the non-coherent combiner output, and may use the orthogonal function index to determine a corresponding complex value from the transformer output.
An envelope detector may calculate the squared magnitude of the transformer output and generate M-ary real values, where M is an integer greater than one. A weighting unit coupled to the envelope detector may estimate a signal quality coefficient from the M-ary real values and generating weighted symbols by multiplying the signal quality coefficient by the corresponding complex value.
Store units may be coupled to the weighting unit for storing the weighted symbol. Also, an averager may be coupled to the store units for averaging the weighted symbols. A controller may be coupled to the non-coherent combiner output, the store units, and the averager, for resetting the store units and for controlling the averager, thereby determining complex channel gain estimation.
Another embodiment of the present invention is a system for performing carrier phase recovery of multi-rate signals which include in-phase and quadrature phase portions. An embodiment of the present invention includes a despreader capable of despreading at least one code from the multi-rate signals. The despreader is also capable of despreading the inphase and quadrature phase signals of the multi-rate signals.
This embodiment also includes a transformer which is coupled to the despreader. The transformer is capable of transforming the in-phase and quadrature phase signals and obtaining a plurality of M-ary complex values, where M is an integer greater than one.
A buffer, non-coherent combiner, and estimator are coupled to the transformer. The buffer is capable of storing the plurality of M-ary complex values. The non-coherent combiner is capable of combining the non-coherent portions of the plurality of M-ary complex values. The estimator is both coupled to the transformer and the non-coherent combiner. The estimator is configured to estimate a complex channel gain from the plurality of M-ary complex values and the non-coherent combining of the plurality of M-ary complex values of all fingers (branches).
In addition a coherent combiner is coupled to the buffer and the estimator. The coherent combiner is configured to perform maximal ratio combining of the stored plurality of M-ary complex values and the channel complex gain estimation. The output of the coherent combiner is a plurality of real value vectors.